The invention relates to a method for simultaneous collection of seismic data from shallow and deep targets, wherein at least two seismic streamers are used, which are towed at different depths, and wherein only one acoustic source is used for the seismic signals.
Marine seismic data are generally collected by the aid of vessels which tow one or more acoustic sources, and one or more listening cables or seismic streamers. Sources and streamers are generally towed at a depth of 5-12 meters below the surface of the sea. A utilized acoustic source emits a pressure wave at regular intervals in time or space, for instance every ten seconds or every 25 meters. Upon passing through the crust of the Earth and being reflected back, the acoustic signals are detected in the seismic streamer. Said streamer is generally between 2 and 6 km long and typically comprises between 120 and 480 hydrophones or hydrophone arrays. Towing of the equipment through the water and movement of the water due to meteorological conditions will also cause acoustic noise to be recorded with seismic signals. It is well-known that noise due to wind and waves is especially bothersome when the seismic streamer is towed in shallow waters. Such noise is often a limiting factor as regards data quality, and it may cause interruption of the collection of seismic data under unfavorable conditions.
Furthermore, it is known that absorption in the crust of the Earth along the path of propagation of the acoustic signals or waves causes attenuation of especially high frequencies. This will in turn result in reflections from shallow targets having a larger proportion of high frequency components than reflections from targets at greater depths. The technique of data collecting is thus typically optimized with respect to one of the targets in the separate survey series. Interest may for instance be focused on a survey of shallow structures or on a survey of deep structures. The technique of collecting has to be adapted to the specific target which is the selected object of the survey. The depth of water in which measuring equipment is located, is a typical parameter which is adapted in consideration of the targets. This will be discussed in more detail below. When it is desirable to survey targets both in shallow waters and in deep waters at the same time, the collecting parameters will be objects of compromise.
The depth of the seismic source and of the streamer, in addition to the above-mentioned attenuation of the acoustic signal or absorption along the signal path of propagation, critically influences the frequency content of the recorded data. At the source some of the emitted energy will be reflected at the surface and will interfere with the remaining emitted waves. This will, in turn, cause some frequency components to be amplified and others to be attentuated, depending on the travel time from the source up to the surface and back again. This additional travel time causes some frequency components to interfere actively, i.e. they are in phase, whereas others interfere destructively, i.e. they are in antiphase. This effect will cause a sine-like modulation of frequency spectra with zero response at sine n for a frequency which is inversely proportional to the towing depth. Such an interference will also occur at the seismic streamer. In order to retain high frequency components from shallow targets right under the sea floor it is, thus, desirable to tow the acoustic source and the seismic streamer right below the surface of the sea. For deeper targets it will not be possible to attain such a high frequency due to the above-mentioned absorption along the propagation path of the signal. Consequently, in this case the equipment is towed at greater depth, which will yield lower frequencies and better acoustic penetration. At the same time, there will be less noise caused by weather conditions at greater depths, as mentioned above.
When it is desirable to survey only particularly shallow geological strata just below the sea floor at the same time as geological strata on more commonly surveyed greater depths, two sources and two seismic streamers are used. Such a method is disclosed in Norwegian Patent Application No. 83 2981 (Newman) filed Feb. 20, 1984. It is a disadvantage of that method that energy from both sources may disturb the recordings of the other. Especially, the source generally used at a deeper level may cause an increased level of noise in recordings of reflections from the weaker source on a shorter seismic streamer which is towed in shallow water. Methods for reducing or eliminating such problems are disclosed, for instance, in U.S. Pat. No. 3,744,021 (Todd). Due to the fact that the shallow additional source of sound is so weak, it may for instance comprise only a single water gun, and the shallow additional seismic streamer is so short, it will only be possible to achieve optimization when surveying very shallow strata just below the sea floor. It is also well-known that a weak acoustic source and a short seismic streamer are not suitable for surveys of deeper strata, for instance strata at depths corresponding to more than 1 second of travel time.
In connection with collecting seismic data by means of at least two seismic streamers, various recording techniques have been proposed, a special object of which was to increase recording capacity on the towing vessel. Such a recording technique is disclosed, for instance, in British Patent Specification No. 1,510,681 (Michon and Staron). As mentioned, there is also the problem of recordings of reflections from the weak source by the short seismic streamer in shallow waters. Methods were therefore proposed to remove such recordings, the so-called ghost effect. In U.S. Pat. No. 3,952,281 and U.S. Pat. No. 3,979,713 (Parrack) a method is proposed, in which recordings on one seismic streamer are delayed and subtracted from recordings on the other in order to eliminate recordings of ghost signals, i.e. downgoing waves. In case of such subtraction data quality is especially sensitive to any deviations of the vertical and horizontal position of the seismic streamers. Additionally, it may readily be demonstrated that the signal/noise ratio deteriorates.
Another method of removing ghost signals according to the above definition is disclosed in GB Patent Application No. 2 081 446 (Ray and Moore). It suggests the use of a seismic streamer having a depth as desired from typically 3 meters at the front end to typically 38 meters at the rear end. Two so-called NMO correction methods are used and are assumed to provide superior quality after summation. The disadvantage of the last-mentioned method is that it is very sensitive to the applied NMO correction which must be more accurate than common prior art and furthermore that noise is generated in the summation process before and after the reflection amplitudes which should be amplified during the summation. With the method proposed according to the last-mentioned application two seismic streamers are used. Combining recordings, an improved signal may be achieved and noise will be attenuated, since it is uncorrelated between the two seismic streamers.